The present invention relates to chairs, and more particularly to chairs capable of dissipating static charges on the chair occupant to the floor.
A wide variety of seating applications require a conductive path between the chair seat and the floor to dissipate static charges which may be carried by the chair occupant. The earliest requirement for such seating known to Applicants was in surgical operating rooms wherein static discharge sparks could ignite ether and other anesthetics commonly used in the early 1960's. Subsequent requirements included electronic component assembly wherein static charges from the worker can have disastrous effects on the parts being assembled. The likelihood of such damage has increased significantly in recent times with the advent of highly sensitive, relatively low voltage electrical components. A third requirement is the computer room of large main-frame computers. Today, grounded seating is desirable, if not required, in many offices with the advent of ever-more-frequent computing equipment such as personal computers.
A variety of grounded chairs has been developed over time in response to this growing need for such seating. In approximately the mid-1970's, Steelcase Inc., the assignee of the present application, provided one such "special order" chair. This chair differed from its conventional chairs by (1) replacing the conventional nonconductive nylon upholstery fabric with a wool or other natural fiber fabric and (2) installing one or more casters having conductive carbon-filled rubber wheels. The metal chair frame, including the pedestal, the column, and the chair iron, completed an electrically conductive path between the chair seat and the floor.
Others also developed grounded seating. Typically, this seating included a conductive upholstery material and conductive casters. At least one prior artisan is believed to have provided a conductive strap between the upholstery fabric and the chair iron to improve the electrical path between the seat and the floor.
As time went on, it became apparent that such seating can be "too conductive" with possible electric shock to the chair occupants. The chair occupant in a low-resistance chair can serve as a portion of a conductive grounded path through the chair. Contact with a relatively high voltage source can cause serious injury to the chair occupant. To overcome this problem, one artisan places resistors in the conductive path between the chair seat and the floor as illustrated in U.S. Pat. No. 4,513,347, issued Apr. 23, 1985, to Wilcox et al, and entitled STATIC PROTECTIVE CHAIR. This chair includes a conductive "drag line" suspended from the chair base and engaging the floor and including resistors between the base and floor. However, this drag line is not aesthetically pleasing. Further, dirt and other contaminants accumulate on the portion of the drag line engaging the floor which changes the resistivity of the conductive path. Third, the construction is undesirably complicated and expensive. Fourth, the resistors can fail, changing the resistance of the chair dramatically.
The Department of Defense has recently published specifications related to electrical properties of materials in its bulletin DOD-HUBK-263. The bulletin defines four categories depending on the surface resistance as follows:
______________________________________ Surface Resistance Protection Category Per Square Provided ______________________________________ Insulative &gt;10.sup.14 NONE. Electrostatic voltage levels generated with these insulators can be extremely high. Anti-Static &lt;10.sup.14 &gt;10.sup.9 Provides protection for moderately sensitive electro- static dissipative items from moderate electrostatic dissipative volt- ages; highly sensi- tive items from low voltages. Static .sup. &lt;10.sup.9 &gt;10.sup.5 Provides protection Dissipative for moderately sensitive electro- static dissipative items from high electrostatic dissipative voltages. Conductive &lt;10.sup.5 Provides protection for highly sensitive electrostatic dissipative items from high electro- static dissipative voltages. ______________________________________
Known chairs are unable to provide the defined "static dissipative" resistance in an aestehtically pleasing, simple, and relatively inexpensive construction.